[llvm] [LoopFlatten] Invalidate SCEV on unflattening bail (PR #211819)

[Arda Serdar Pektezol via llvm-commits]

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pektezol wrote:

Ah I see, so it's better to move this under CanWidenIV before canflatten?
```cpp
  // Save the old/narrow induction phis, which we need to ignore in CheckPHIs.
  FI.NarrowInnerInductionPHI = FI.InnerInductionPHI;
  FI.NarrowOuterInductionPHI = FI.OuterInductionPHI;

  SE->forgetLoop(FI.OuterLoop);
  SE->forgetBlockAndLoopDispositions();

  // After widening, rediscover all the loop components.
  return CanFlattenLoopPair(FI, DT, LI, SE, AC, TTI);
}
```

https://github.com/llvm/llvm-project/pull/211819